In an electronic apparatus of the related art such as a server apparatus or a personal computer (PC), a fan (blower) that releases heat outside by sending air into the apparatus is sometimes installed to prevent the internal temperature of the apparatus from being increased by heat generation of a processor or the like.
In recent years, to effectively use the limited installation environment, the size of an air duct provided in the electronic apparatus is sometimes reduced so as to decrease the size of the electronic apparatus, and only a small-sized fan can be installed in an increasing number of cases. Accordingly, to sufficiently cool the interior of the electronic apparatus even when only a small-sized fan can be installed, various measures are adopted, for example, a high air blowing pressure is generated by increasing the rotation speed of the fan, or a plurality of fans are arranged in tiers.
FIGS. 8A and 8B illustrate cases in which a plurality of fans are arranged in tiers. FIG. 8A illustrates an example of an electronic apparatus in which two fans are arranged in tandem, and FIG. 8B illustrates an example of an electronic apparatus in which four fans are arranged in tandem and in parallel.
Referring to FIG. 8A, a fan unit 320a including two fans 321a and 321b arranged in tandem is provided in an air duct 310 of an electronic apparatus 300. In the electronic apparatus 300, air is blown to heating elements 330a and 330b, such as processors, in the electronic apparatus 300 by rotating the fans 321a and 321b, so that the heating elements 330a and 330b are cooled by the air serving as a refrigerant. In contrast, the electronic apparatus 300 sometimes includes a fan unit 320b in which four fans 321c to 321f are arranged in tandem and in parallel, as illustrated in FIG. 8B.
In this way, even when the air duct is narrow, a sufficient amount of air to cool the heating elements can be obtained by arranging the fans in tiers. Here, of the fans arranged in tiers along the air duct, a fan provided on a side where air is introduced from the outside, for example, the fans 321a, 321c, and 321e, are referred to as a front fan, and a fan provided on a side where the air is blown toward the heating elements 330a and 330b, for example, the fans 321b, 321d, and 321f, is referred to as a rear fan.
In the fan unit including the front fan and the rear fan, if one of the front fan and the rear fan stops because of a breakdown, fans of the faulty fan cause a high airflow resistance, and obstruct air blowing to the heating elements. This may hinder sufficient cooling of the heating elements.
That is, when the front fan 321a and the rear fan 321b normally operate, an airflow generated by the fan unit 320a is smoothly introduced into the apparatus, as illustrated in FIG. 9A. However, for example, if the rear fan 321b breaks down and stops, as illustrated in FIG. 9B, an airflow generated by the front fan 321a is blocked by the rear fan 321b, and this seriously reduces the amount of air to be introduced into the apparatus.
To cope with such a case, for example, Japanese Registered Utility Model No. 3086497 describes that, when one of the fans breaks down, the pressure of air to be introduced into the apparatus is ensured by increasing the rotation speed of the other fan, thereby reliably cooling the heating elements.
Unfortunately, in the related art, when the rotation speed of the fan is increased, the fan makes more noise.
That is, the fan makes noise (wind noise) due to an eddy of air formed near the fans during operation. Since noise made by operation of the fan increases as the amount of blowing air increases, when the amount of blowing air is increased by increasing the rotation speed of the fan, noise also increases. More specifically, it is said that the amount of noise made by rotation of the fan is proportional to the fifth or sixth power of the rotation speed of the shaft of the fan.
For example, when the rear fan 321b breaks down, as illustrated in FIG. 9B, it is assumed that there is a need to increase the blowing pressure of the front fan 321 by 2.6 times in order to compensate for the air blowing pressure of the rear fan 321b and the loss of blowing pressure of the rear fan 321b due to the airflow resistance. In this case, the rotation speed of the front fan 321a is set to 1.6 times the rotation speed in a normal state. As a result, noise made by rotation of the front fan 321a becomes larger by about 10.5 to 16.8 dB(A) than when the front fan 321a rotates at the normal rotation speed. As for the two fans, that is, the front fan 321a and the rear fan 321b, noise is increased by 7.5 to 13.8 dB(A).
Since noise of the fan increases as the rotation speed of the shaft of the fan increases in this way, when one of the front fan and the rear fan breaks down and the rotation speed of the other normal fan is increased to ensure a sufficient amount of blowing air, a large noise is made. In particular, recent electronic apparatuses have been installed not only in a special place such as a computer room, but also in general offices, and consciousness of noise reduction has been raised. Accordingly, there is a demand to minimize noise of the fan.